The actual dimensions don’t matter as much as how many people fit comfortably.
Open bench seating will fit more people in a hot tub due to the lack of sculpted contours separating each user. I call it “negotiable seating”, where you can scooch over to make more room to pack in as many people as possible.
Compare that to a more aggressively-sculpted hot tub with each person in their own “pod”, where they may need to stand up to relocate.
Sculpted vs open bench seating
Also, some hot tubs have a wider shelf around the perimeter, providing ample shelf space for cups and more comfortable sill-sitting, but this results in less interior space.
Narrow vs wide sill perimeter
Also, keep in mind that people also have legs, not just butts. A six-seater hot tub may have enough seats for six butts, but not enough room for twelve legs.
Large vs small foot well